Protective arrangement for semiconductor rectifiers



A. C. STUMPE Jan. 3, 1967 PROTECTIVE ARRANGEMENT FOR SEMICONDUCTORRECTIFIERS Filed May 16, 1965 Jnvenlor:

AUGUST CHRIST/AN STUMPE .a /WMAIL QW Aflomeys United States Patent3,296,518 PROTECTIVE ARRANGEMENT FOR SEMI- CONDUCTOR RECTIFIERS AugustChristian Stumpe, Frankfurt am Main, Germany,

assiguor to Licentia Patent-Verwaltungs-GmbH, Frankfurt am Main, GermanyFiled May 16, 1963, Ser. No. 280,818

Claims priority, application Germany, May 23, 1962,

L 42,051 1 Claim. (Cl. 321-14) The present invention relates to aprotective arrangement for semiconductor elements such as rectifiers.

Utilization of semiconductor elements both as controlled rectifiers andas uncontrolled rectifiers requires the provision of protective means inorder to prevent such heat sensitive semiconductor element, usually agermanium or silicon single crystal, from undue heating by overload orshort-circuit currents. Usually,a fuse is connected in series with asemiconductor rectifier element or with a group of such elements. Thefuse must meet particular requirements in order to provide forsufiicient protection for the rectifier element or elements. Onecondition, of course, is that the response value for the fuse must besomewhat below the permissible current maximum of the semiconductor.Another requirement is that the arc voltage in the fuse after responsethereof, is to be below the maximum voltage permissible for the othercircuit elements of the network containing the rectifier.

The presently known fast acting fuses meet these requirements in case oflow short-circuit currents and therefrom resulting long response time ofthe fuse. However, after the fusible conductor pertaining to the fuseelement has melted down, there still remains an arc extending from andbetween the non-melting fuse electrodes whenever there are inductancesor capacitances connected in the electric circuit rectifier networkfeeding stored energy as electric current into this fuse are whichcurrent flows also through the semiconductor rectifier or rectifiers.This discharge current flows as long as there is electromagnetic orelectrostatic energy stored in the inductances or capacitancesrespectively suflicient to maintain the fuse arc and until this energypermitted to be discharged has been converted into heat. This heat isdeveloped partially in the arc, partially in the ohmic resistances ofthe network not being short-circuited and partially in the semiconductorrectifier body. Since the amount of heat necessary to melt the fusibleconductor is constant, every additional energy is at least partiallydirectly discharged into the semiconductor for conversion into heattherein, and the fuse then being in effect burnt out is powerless toprevent this resulting development of heat in the semiconductor body.

It is an object of the present invention to eliminate the heating up ofsemiconductor rectifiers in case of shortcircuits after a fuse burnsout.

It is another object of the present invention to provide for a new andimproved protective arrangement for semiconductor rectifiers connectedin circuit with inductances or capacitances.

According to one aspect of the invention in a preferred embodimentthereof, it is suggested to connect a fusible element directly in serieswith the semiconductor rectifier element to be protected and a spark gapmeans is shunted "ice across fusible element and rectifier element whilebeing physically located in the vicinity of the fusible element.Whenever the fusible element melts because of a shortcircuit or of anoverload current, an arc is formed at the fuse ionizing the regionbetween the electrodes pertaining to the spark gap means so as to firethe latter. Upon firing of the spark gap means, the still flowingelectric current is commutated away from fuse and rectifier and flowsnow through the spark gap means.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which is regarded as theinvention, it is believed that the invention, the objects, and featuresof the invention and further objects, features and advantages thereofwill be better understood from the following description taken inconnection with the accompanying drawing in which:

FIGURE 1 illustrates a circuit network diagram first embodiment of theinvention; and

FIGURE 2 illustrates a circuit network diagram of a second embodiment ofthe invention modified as compared with the first embodiment.

Proceeding now to the detailed description of the drawing, in FIGURE 1there is illustrated a first embodiment of the invention.

A transformer 1 has a primary winding 11 connected to an AC. voltagesource such as the mains '8. A secondary winding 12 of transformer 1provides for the necessary A.C. supply voltage level. There is providedin the circuit network for the secondary winding a semiconductorrectifier 3 preferably of the controlled type and being connected inseries with a load 4 as well as with a fuse 2.

An inductance 6 illustrated as series connected circuit element is torepresent inductivity of the circuit outside of the load itself. Inparticular inductance 6 represents that inductivity in the secondarytransformer circuit, which will remain in series circuit connection withrectifier 3 in case of a shortacircuit across the load 4.

The particular series circuit connection of fuse 2 and of rectifier 3 isshunted by a spark gap 5 dimensioned so as to fire whenever an arcappears in the fuse after the fuse element has melted. In particular,spark gap- 5 is located in the vicinity of the fusible element of fuse 2so that an arc in the fuse 2 ionizes the space between the electrodespertaining to the spark gap 5 so that the latter can fire indeed.

During normal operation, the voltage drop across fuse 2 added to thevoltage drop in forward direction across rectifier 3 is insufficient tofire spark gap 5. Also, the blocking voltage in case of normal operationacross rectifier 3 is insufficient to fire spark gap 5. In either caseno arc is being formed in fuse 2.

Now the case of short-circuit across the load 4 or any other overloadcurrent through semiconductor rectifier 3 is to be considered.

of a Upon occurrence of such short-circuit, the short-circuit Theresponse of fuse 2 is carried out by melting of the meltable fuseconductor therein.

The melting of the fusible element of fuse 2 is determined by theso-called melt integral fi dt which is the time integral over the squareof the current values occurring during melting through the conductor anddetermining the amount of electric and heat energy required to melt thefusible element. This integral is approximately constant in the range ofhigh short-circuit current values and short melting periods.

In order to develop more fully purpose and function of the inventivedevice it shall first be assumed that spark gap 5 were not employed.

After the fusible element or conductor has been melted so as tophysically interrupt the conductive current path, of the rectifiernetwork there still is stored electrical energy in inductance 6 whichwill discharge through the circuit and accordingly an arc will be formedacross the otherwise interrupted current path in the fuse 2. Thiscurrent will flow as long as there is sufiicient energy stored to feedand maintain the arc in the fuse 2. Hence, the entire current passedthrough the electric circuit network results in an overall integral fidt well above the aforedefined melting integral. The meltintegral isdetermined primarily by the properties of the fusible element, whereasthe overall time integral over the square of the current is determinednot only by the characteristics of the circuit elements employed butalso by the initial short-circuit conditions and the amount of energystored in and dischargeable from the circuit network inductances at thetime when the short-circuit occurs.

This resulting overall integral value increases, of course, with thesquare of the current permitted to flow through the are at the fuse andmight have a multiple value as that of the melting integral. Also, theentire period of time during which a short-circuit and discharge currentstill flows across the fuse arc is much longer than that needed to meltthe fusible element.

The semiconductor 3 is being passed through by this extendedshort-circuit and discharge current. The maximum permissible load for asemiconductor element itself is likewise determined by a particularintegral ji dt which is of approximately constant value as to thepermissible maximum for periods of time below 10 ms.

It is thus apparent, that no problem exists, if there were noenergy-storing inductances (or capacitances), so that the overallintegral for the existing conditions then is equal to or only slightlyabove the melt integral, and both integral values remain well below thepermissible semiconductor current integral value. The usual inductance 6of such a rectifier network, however, is expected to produce an overallintegral value well above the melt integral and thus possibly above thepermissible semiconductor current integral value.

It is thus apparent that the fuse 2 alone as aforedescribed does notprotect suificiently the semiconductor rectifier 3, since the fuse, infact, has no control over the extended current flow through an arc inthe fuse, after fuse burn out which current flow extends the developmentof heat in the semiconductor rectifier in an uncontrolled manner.Moreover, the inductance 6 will discharge electric energy until all itsenergy has been converted into heat regardless what effect this has onthe semiconductor. Some heat, of course, is produced and dissipated inthe are formed in the fuse, another sizable portion of heat, however, isdeveloped in the rectifier body itself and this amount of heat dependsprimarily on the energy content stored in the inductance 6 and to bedischarged fully therefrom.

The invention now provides for a spark gap 5 shunted across fuse 2 andrectifier 3 and in such physical vicinity of the fusible element in fuse2, so that the spark gap fires whenever an arc develops across fuse 2.When spark gap 5 thus fires, the short-circuit current is com- .4mutated to now flow over spark gap 5 thus bypassing semiconductorrectifier element 3. The only current still flowing throughsemiconductor element 3 is that necessary to complete the melt integralso that the fuse 2 receives that amount of energy necessary to melt downthe conductor therein completely, but no more electric current than thatflows into the fuse and through rectifier 3.

It is apparent that with the provision according to the invention andbefore and after the aforedescribed current commutation takes place, theentire current flowing through semiconductor rectifier 3 in case of ashortcircuit is only determined by the melt integral of fuse 2, which iswell known and predeterminable for the fuse and will be maintained belowthe maximum permissible semiconductor integral. This is true regardlesshow large the short-circuit current is and how long it takes theinductance 6 to discharge its electromagnetic energy, since everyextended current flow above that required to complete the fuse meltintegral is being commutated across the spark gap 5. Thus, thesemiconductor rectifier is now adequately protected jointly by the fuse2 and the spark gap 5.

The spark gap 5 performs an additional function; namely, it fires alsoin case of voltage peaks in the secondary winding 12 which peaks are perse insufiicient to cause fuse 2 to respond, but which peaks might breakthrough the semiconductor.

FIGURE 2 illustrates a further embodiment of the invention. First thereare provided the aforedescribed elements such as transformer secondarywinding 12, inductance 6, load 4 and rectifier 3 interconnected also asaforedescribed. Connected in series with rectifiel's is an ordinary fuse21.

In addition, there is now provided a circuit element 25 composed of aspark gap 5 and of a meltable conductor 2 disposed in the vicinity ofgap 5. The meltable conductor 2 is directly connected in parallel tofuse 21 so that elements 2 and 21 together could be considered asconstituting one fuse with two parallel conductors. The spark gap 5bridges all the elements 21, 2 and 3.

In this embodiment one can use an ordinary fuse 21 conveniently locatedin the apparatus right at the one output terminal of secondary winding12, while element 25 is designed to have a fusible conductor 2 which incase of fuse response melts and forms an are directly adjacent theelectrodes forming the spark gap 5 so as to ionize the gap 5.Preferably, element 25 is cased into a housing common for conductor 2 aswell as gap 5. Fuse 21 will have a slightly higher response current thanconductor 2, so that conductor 2 will start to respond (fuse) thusincreasing its resistance and shifting now some of the current to fuse21 so that now the latter will also respond.

In both embodiments spark gap 5 is thus determined by the followingrequirements:

Spark gap 5 must not fire at the reverse or blocking voltage acrossrectifier 3 at normal operating voltage and current levels, nor must itbe fired by the voltage drop when current flows in forward directionthrough rectifier 3 and fuse 2 (and/or 21).

The gap 5 must fire when an arc is being formed across fuse 2 which arcthen ionizes the gap between the electrodes of spark gap 5. This firingmust be more or less independent from the voltage drop across the arc offuse 2 plus the voltage drop across rectifier 3 in case of ashort-circuit current.

Finally, spark gap 5 should fire in case of temporary voltage peaksacross secondary winding 12 insufiicient to cause melting of the fuse 2.

It has been found advantageous to use silver as fusible conductor infuse 2 and this fusible conductor is being placed at a distance fromspark gap within the range of 0.5 to 3 mm., preferably 1 mm.

The invention is not limited to the embodiments described above but allchanges and modifications thereof not constituting departures from thespirit and scope of the invention are intended to be covered by thefollowing claims:

What is claimed is:

Protective arrangement for semiconductor rectifiers connected between anAC. voltage source and a load comprising: a fuse connected in serieswith the semiconductor rectifier, a fusible element connected parallelto said fuse and in series to said rectifier; and a spark gap meansconnected across said fusible element and said rectifier and beingphysically located near said fusible element so that an are producedupon melting of said fusible element fires said spark gap means.

3,160,805 12/1964 Lawson.

FOREIGN PATENTS 905,188 9/ 1962 Great Britain.

JOHN F. COUCH, Primary Examiner.

W. H. EEHA, Assistant Examiner.

